In order to reduce interconnect capacitances in high performance and high frequency processes, air bridges are often used. A typical air bridge is formed using a second layer of interconnect metal deposited and patterned over a sacrificial material. The sacrificial material is later removed to leave a metal line surrounded by air rather than a dielectric, such as oxide. Parasitic capacitances to the substrate and other metal lines is thus reduced since air has a lower dielectric constant than do solid insulators such as silicon dioxide or silicon nitride.
However, traditional air bridge manufacturing techniques and structures have several disadvantages. The length of an air bridge is often limited by flexure of metal between two vias. So, relatively long air bridges can only be manufactured by stitching together multiple lengths of short air bridges. Another problem is that circuits fabricated with air bridges cannot be passivated. In a normal process, the passivation layer is deposited on top of an integrated circuit. Typical passivation layers are silicon oxide or silicon nitride. However, for air bridge structures, the passivation layer has to be omitted otherwise the passivation layer will fill the air under the bridge and thereby increase the capacitance of the air bridge or damage the bridge itself. Plastic packaging is also precluded for the same reasons.
Accordingly, there has arisen a need for air bridges that can be made of longer lengths of metal than are available in air bridges of the prior art and also for air bridges that can be incorporated into integrated circuits which are passivated and/or in plastic packages.